1. Field of Invention
This invention relates to a process for forming medical implants of cross-linked ultrahigh molecular weight polyethylene having an improved balance of wear properties and oxidation resistance.
2. Description of Related Art
It is known in the art that ultrahigh molecular weight polyethylene (UHMWPE) can be cross-linked by irradiation with high energy radiation, for example gamma radiation, in an inert atmosphere or vacuum. Exposure of UHMWPE to gamma irradiation induces a number of free-radical reactions in the polymer. One of these is cross-linking. This cross-linking creates a 3-dimensional network in the polymer which renders it more resistant to adhesive wear in multiple directions. The free radicals formed upon irradiation of UHMWPE can also participate in oxidation which reduces the molecular weight of the polymer via chain scission, leading to degradation of physical properties, embrittlement and a significant increase in wear rate. The free radicals are very long-lived (greater than eight years), so that oxidation continues over a very long period of time resulting in as much as a 5-fold increase in the wear rate as a result of oxidation over a period of about 5 years. As such, the wear rate of traditionally irradiated materials is significantly greater than unirradiated materials.
Sun et al. U.S. Pat. No. 5,414,049, which patent is hereby incorporated by reference, discloses a process for improving the oxidation resistance of medical implants which have been sterilized with radiation. In a preferred embodiment, a raw polymeric material is obtained by forming a virgin resin powder from which air and moisture have been removed prior to the forming process. The forming process, e.g. ram extrusion or compression molding of the powder is also preferably carried out in an inert low oxygen atmosphere. A medical implant is formed from the raw material, e.g. an olefinic material such as UHMWPE, and is sealed in an oxygen impermeable package in an oxygen reduced non-reactive atmosphere and radiation sterilized, followed by heating the sterilized packaged implant at a temperature of between 37.degree. C. and the melting point of the olefinic material. The heating step forms cross-links between remaining free radicals formed during the radiation sterilization step, thus improving the oxidation resistance of the material. If the annealing temperature is too high, the thermal treatment can cause distortion of the UHMWPE implant, which is undesirable in orthopedic end-uses where the implants are generally machined or molded to close tolerances. If the temperature profile during annealing is adjusted to lower temperatures to avoid distortion of UHMWPE implants, incomplete extinguishing of the free radicals generally occurs and the UHMWPE oxidizes upon exposure to air or moisture.
Hyun et al. published International Application WO 96/09330, which application is hereby incorporated by reference, discloses a process for forming oriented UHMWPE materials for use in artificial joints by irradiating with low doses of high-energy radiation in an inert gas or vacuum to cross-link the material to a low degree, heating the irradiated material to a temperature at which compressive deformation is possible, preferably to a temperature near the melting point or higher, and performing compressive deformation followed by cooling and solidifying the material. The oriented UHMWPE materials have improved wear resistance. Medical implants may be machined from the oriented materials or molded directly during the compressive deformation step. The anisotropic nature of the oriented materials may render them susceptible to deformation after machining into implants.
Salovey et al. published European Application EP 722973, which application is hereby incorporated by reference, discloses a method for enhancing the wear-resistance of polymers, including UHMWTPE, by crosslinking them via irradiation in the melt or using peroxide or similar chemical agents.
The process of the present invention eliminates the problem of thermal distortion of the implant by irradiating and thermally treating a consolidated UHMWPE material prior to forming an implant therefrom. By separating the irradiation cross-linking step from the sterilization step, the current invention allows one to use lower levels of irradiation for cross-linking than would be effective for sterilization of UHMWPE. Additionally, the heat treatment step following irradiation may be performed at temperatures near or above the melting point, which would distort the shape of a machined or molded implant which has been packaged for final sterilization. Heat treatment near or above the melting point results in improved molecular mobility, elimination of free radicals in the crystalline regions of the polymer which cannot occur at temperatures below the melt, and increased cross-linking and reduced oxidation in aged samples. When heat treatment is carried out at lower temperatures, incomplete quenching of free radicals results in residual oxidation upon aging., By separating the irradiating and annealing steps, the process of the current invention also avoids the need to irradiate in the melt, which is difficult to achieve on a commercial scale. An additional object of the current invention is to prepare a medical implant of cross-linked UHMWPE having improved wear properties without the use of chemical cross-linking agents. Medical implants formed from the cross-linked UHMWPE material of the current invention may be packaged in an air-permeable package and sterilized using non-irradiative methods such as gas plasma or ethylene oxide, eliminating the need to package the implants in an inert atmosphere. The cross-linked UHMWPE of the current invention can also be used in nonmedical applications requiring high wear resistance.